Real-image finder

ABSTRACT

An indication-within-finder free from ghost light is provided at a minimal cost even in the case of a pentagonal prism or the like. The finder includes an objective system ( 61 ) having a positive refracting power, an image-inverting system including a roof prism ( 62 ) and a pentagonal prism ( 63 ), and an ocular system ( 64 ) having a positive refracting power. An object image is formed on an intermediate image plane ( 65 ) by the objective system ( 61 ) and the roof prism ( 62 ). The object image is viewed with the ocular system ( 64 ) through the pentagonal prism ( 63 ). The position of the intermediate image plane ( 65 ) is approximately coincident with the entrance surface of the pentagonal prism ( 63 ). A deflecting member ( 67 ) projecting in a wedge shape is provided on the entrance surface of the pentagonal prism ( 63 ). Light rays ( 68 ′) passing through the deflecting member ( 67 ) are deflected by refracting surfaces ( 67 ′ and  67 ″). Therefore, the rays ( 68 ′) do not reach a viewer&#39;s pupil, but blacked-out indications are seen.

BACKGROUND OF THE INVENTION

The present invention relates to an indication display device of afinder optical system used in a camera for photography or an electroniccamera.

A finder for a camera needs indication display within the field of viewto display an indication of a distance measurement range forautofocusing and an indication for parallax correction in photography atthe closest focusing distance. Conventionally, a real-image finder isprovided with an indication display member near an intermediate imageformation plane. Known indication display methods include a methodwherein incident light is blocked to display an indication, and a methodwherein a wedge-shaped deflecting member is provided on the surface ofan optical element of a Porro prism to deflect incident light in adirection in which it does not reach a viewer's pupil, as disclosed inGazette Containing the Japanese Patent No. 2629690.

However, the method wherein incident light is blocked requires extraprocessing, e.g. vapor deposition onto a transparent member. This causesan increase in the cost. In the case of the method wherein incidentlight is deflected, if an indication display member is formed as amolded part, it is unnecessary to subject each individual parts to extraprocessing, and there is no increase in the cost. However, in the caseof a prism in which an entrance surface and an exit surface are disposedclose to each other as in a pentagonal prism, for example, deflectedrays may be totally reflected by the exit surface to reach the viewer'spupil as light still having a strong intensity, thus causing ghostlight. Rays deflected by a deflecting member for indication deviate froma visual field viewing light path but are still directed toward thepupil. In a Porro prism such as that shown in FIG. 1, an entrancesurface 11 is surrounded with a first reflecting surface 12 and threediffusing surfaces 13. Therefore, most of deflected rays impinge on thediffusing surfaces 13 to become diffused light of weak intensity.Accordingly, only a slight amount of light reaches the viewer's pupiland is not recognized as ghost light. In the case of a pentagonal prismsuch as that shown in FIG. 2, light that travels along a normal lightpath enters the prism through an entrance surface 21 and is successivelyreflected by reflecting surfaces 23 and 23′ to exit from an exit surface22. However, the exit surface 22 has a mirror surface not included inthe normal light path in the neighborhood of the entrance surface 21.Therefore, as shown in part (a) of FIG. 4, deflected rays 40 may betotally reflected by an exit surface 41 (22) and reflected by a firstreflecting surface 42 and a second reflecting surface 43 to reach aviewer's pupil without being diffused. In this case, a bright ghostimage having a size equivalent to the indication is undesirablyobserved, giving rise to a problem.

SUMMARY OF THE INVENTION

In view of the above-described problems with the prior art, an object ofthe present invention is to provide an indication-within-finder madefree from ghost light at a minimal cost even in the case of a prismhaving a mirror surface not included in the normal light path in theneighborhood of an entrance surface thereof as in a pentagonal prism.

To attain the above-described object, the present invention provides areal-image finder including an objective system having a positive power,and an image-inverting system including a prism. The real-image finderfurther includes an ocular system having a positive power. A deflectingmember is placed near an intermediate image plane formed by theobjective system to deflect incident light rays with respect to a visualfield viewing light path. A prism is placed closer to the ocular systemthan the intermediate image plane. The prism has a mirror surface notincluded in a normal light path in the neighborhood of an entrancesurface thereof. Light rays deflected by the deflecting member andtotally reflected by the mirror surface are blocked or scattered beforereaching the ocular system.

It is desirable that the deflected light rays should satisfy thefollowing condition (1):

 20°<ω  (1)

where ω is an angle formed between the deflected light rays and themirror surface.

It is more desirable that the deflected light rays should satisfy thefollowing condition (1′):

20°<ω<50°  (1′)

It is also desirable that the deflecting member should have a pair ofrefracting surfaces, and the refracting surfaces should satisfy thefollowing condition (2):

ω′<40°  (2)

where ω′ is an angle formed between each of the refracting surfaces andthe mirror surface.

It is more desirable that the deflecting member should have a pair ofrefracting surfaces, and the refracting surfaces should satisfy thefollowing condition (2′):

10°<ω′<40°  (2′)

It is also desirable that the deflecting member should have a pair ofrefracting surfaces, and the refracting surfaces should satisfy thefollowing condition (3):

40°<θ<100°  (3)

where θ is a vertex angle formed between the pair of refractingsurfaces.

It is more desirable that the deflecting member should have a pair ofrefracting surfaces, and the refracting surfaces should satisfy thefollowing condition (3′):

50°<θ<90°  (3′)

In addition, the present invention provides a real-image finderincluding an objective system having a positive power, and animage-inverting system including a prism. The real-image finder furtherincludes an ocular system having a positive power. A deflecting memberis placed near an intermediate image plane formed by the objectivesystem to deflect incident light rays with respect to a visual fieldviewing light path. A prism is placed closer to the ocular system thanthe intermediate image plane. The prism has a mirror surface notincluded in a normal light path in the neighborhood of an entrancesurface thereof. The deflecting member has a pair of refractingsurfaces. A ridge formed between the pair of refracting surfaces is notparallel to the mirror surface.

In addition, the present invention provides a real-image finderincluding an objective system having a positive power, and animage-inverting system including a prism. The real-image finder furtherincludes an ocular system having a positive power. A deflecting memberis placed near an intermediate image plane formed by the objectivesystem to deflect incident light rays with respect to a visual fieldviewing light path. A prism is placed closer to the ocular system thanthe intermediate image plane. The prism has a mirror surface notincluded in a normal light path in the neighborhood of an entrancesurface thereof. The deflecting member has a diffusing surface.

It is desirable that the deflecting member should be provided on theentrance surface of the prism.

It is desirable that the prism should be a pentagonal prism.

It is desirable that the prism should be formed by injection molding ofa plastic material.

The reasons for adopting the above-described arrangements, together withthe functions thereof, will be described below.

If the system is arranged to block or diffuse 80% or more of light raysreflected by the mirror surface, which is not included in the normallight path, after being deflected by the deflecting member, ghost lightgives rise to no problem even at a position other than the pupilposition assumed in design. The ocular system, which extends from theintermediate image plane to the finder exit part, can be arranged in acompact form by allowing the deflected light rays to impinge at a deepangle on the mirror surface, which is not included in the normal lightpath. Visual field rays near the intermediate image plane areapproximately telecentric, i.e. approximately parallel to the opticalaxis. Therefore, in the case of a finder with a small-sized ocularsystem, as shown in FIG. 5, if the angle ω formed between a mirrorsurface 51 and a light ray 50 deflected by a deflecting member 54 issmall, the ray 50 is totally reflected by the mirror surface 50 and thenpasses through an ocular system 52 to reach a viewer's pupil with astrong intensity without being blocked. Conversely, if the angle ωformed between a deflected ray 53 and the mirror surface 51 is large, itbecomes more likely that the ray 53 will not satisfy the condition fortotal reflection at the mirror surface 51. Even if the ray 53 is totallyreflected, there is a strong probability that the reflected ray 53 willbe blocked before passing through the ocular system 52. For the reasonsstated above, it is preferable for the deflected light rays to satisfythe following condition.

20°<ω  (1)

where ω is an angle formed between the deflected light rays and themirror surface.

If the angle ω is not larger than the lower limit of the condition (1),the amount of deflection is excessively small, and it becomes morelikely that ghost light will occur.

There is also a likelihood that deflected light rays will pass throughthe mirror surface instead of being reflected thereby and exit from theocular lens as stray light. Therefore, it is even more desirable thatthe deflected light rays should satisfy the following condition (1′):

20°<ω<50°  (1′)

In a case where the deflecting member is, as shown in FIG. 3, arefracting member 31, e.g. a small prism, if the angle formed betweeneach refracting surface 32 and a mirror surface 33 is large [see part(a) of FIG. 3], the amount of deflection is small. Consequently, adeflected light ray 34 impinges on the mirror surface 33 at a shallowangle unfavorably. Accordingly, it becomes more likely that ghost lightwill occur as shown in part (a) of FIG. 4. Conversely, if the angleformed between each refracting surface 32 of the refracting member 31and the mirror surface 33 is small [see part (b) of FIG. 3], the amountof deflection is large, and the deflected light ray 34 impinges on themirror surface 33 at a deep angle. Accordingly, the possibility of ghostlight occurring reduces as shown in part (b) of FIG. 4. For the reasonsstated above, it is preferable that the refracting member serving as adeflecting member should satisfy the following condition (2):

ω′<40°  (2)

where ω′ is an angle formed between each refracting surface and themirror surface.

If the angle ω′ is not smaller than the upper limit of the condition(2), the amount of deflection is excessively small, and it becomes morelikely that ghost light will occur.

There is also a likelihood that deflected light rays will pass throughthe mirror surface instead of being reflected thereby and exit from theocular lens as stray light. Therefore, it is even more desirable thatthe refracting member serving as a deflecting member should satisfy thefollowing condition (2′):

10°<ω′<40°  (2′)

In addition, it is preferable that the refracting member serving as adeflecting member should satisfy the following condition (3):

40°<θ<100°  (3)

where θ is a vertex angle formed between the pair of refractingsurfaces.

If the angle θ is not larger than the lower limit of the condition (3),as shown in part (c) of FIG. 3, incident light 34 deflected by onerefracting surface 32′ of a pair of refracting surfaces 32′ and 32″ maybe totally reflected by the other refracting surface 32″ and thusdeflected again, remaining as a light ray 44 of shallow angle as shownin part (c) of FIG. 4. If the angle θ is not smaller than the upperlimit of the condition (3), there may be light rays that undesirablyenter the ocular system directly without being satisfactorily refractedby the refracting member 31. Consequently, displayed indications cannotbe completely blackened. Thus, it becomes impossible to perform theintended indication display.

In a case where the prism is formed by injection molding of a plasticmaterial, it is preferable that the refracting member serving as adeflecting member should satisfy the following condition (3′):

50°<θ<90°  (3′)

If the angle θ is not larger than the lower limit of the condition (3′),the refracting member 31 becomes excessively thin. Consequently, itbecomes difficult to charge the plastic material, and moldabilitydegrades. If the angle θ is not smaller than the upper limit of thecondition (3′), there may be light rays that enter the ocular systemwithout being satisfactorily refracted by the refracting member 31.

Even in the case of a prism having a mirror surface not included in thenormal light path in the neighborhood of an entrance surface thereof, ifa ridge formed between a pair of refracting surfaces that constitute arefracting member serving as a deflecting member is not parallel to themirror surface, no light rays are deflected toward the mirror surface.Therefore no ghost light occurs.

Furthermore, if deflected light rays are diffused by the deflectingmember itself, even if light rays reach the viewer's pupil, the lightintensity is weak, and ghost light is inconspicuous. In a case where therefracting surfaces are mirror surfaces, even if the deflecting memberis arranged as stated above, it is still likely that stray light due tolight, exclusive of visual field light, e.g. light internally reflectedby the objective lens frame, will reach the viewer's pupil as ghostlight. However, if the refracting surfaces are formed in the shape ofdiffusing surfaces, it is possible to reduce the quantity of stray lightundesirably reaching the viewer's pupil.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification.

The invention accordingly comprises the features of construction,combinations of elements, and arrangement of parts which will beexemplified in the construction hereinafter set forth, and the scope ofthe invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a conventional Porro prism.

FIG. 2 is a perspective view showing a conventional pentagonal prism.

FIG. 3 is a diagram illustrating the deflection of light rays by arefracting member serving as a deflecting member.

FIG. 4 is a diagram showing the way in which ghost light occursdepending upon the deflection angle of light rays.

FIG. 5 is a development of a pentagonal prism.

FIG. 6 is a diagram showing a real-image finder according to a firstembodiment of the present invention.

FIG. 7 is a diagram showing a deflecting member in a second embodimentof the present invention.

FIG. 8 is a diagram showing a deflecting member in a third embodiment ofthe present invention.

FIG. 9 is a diagram showing a deflecting member in a fourth embodimentof the present invention.

FIG. 10 is a diagram showing a deflecting member in a fifth embodimentof the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the real-image finder according to the present inventionwill be described below.

As shown in part (a) of FIG. 6, a real-image finder according to a firstembodiment of the present invention includes, in order from an objectside thereof, an objective system 61 having a positive refracting power,an image-inverting system including a roof prism 62 and a pentagonalprism 63, and an ocular system 64 having a positive refracting power.The ocular system 64 includes an ocular lens 64′ and an ocular window64″. An object image is formed on an intermediate image plane 65 by theobjective system 61 and the roof prism 62. The object image is viewedwith the ocular system 64 through the pentagonal prism 63. Thepentagonal prism 63 has an entrance surface that is convex toward theobject side. The position of the intermediate image plane 65 isapproximately coincident with the entrance surface of the pentagonalprism 63. As shown in part (b) of FIG. 6, which is a sectional view, adeflecting member 67 projecting in a wedge shape is provided on theentrance surface of the pentagonal prism 63 to display an indication forparallax correction and an indication of an AF distance measurementrange. The deflecting member 67 has a linear configuration extending ina direction normal to the plane of the figure. Light rays 68 that do notpass through the deflecting member 67 reach a viewer's pupil as visualfield viewing light. Light rays 68′ that pass through the deflectingmember 67 are deflected by refracting surfaces 67′ and 67″ and thereforedo not reach the viewer's pupil. Accordingly, the viewer seesblacked-out indications as shown in part (c) of FIG. 6. Referencenumeral 69 denotes a parallax correction indication. Reference numeral69′ denotes an indication of an AF distance measurement range. In thiscase, there is a possibility that the rays 68′ may impinge directly onan exit surface 63′ and be totally reflected thereby to reach theviewer's pupil as ghost light. In this embodiment, however, the angle ω′between the refracting surface 67′ of the deflecting member 67 and theexit surface 63′ is set to satisfy the above-described condition (2).Therefore, the totally reflected rays are blocked in the vicinity of theocular lens 64′ and do not reach the viewer's pupil. Accordingly, ghostlight is unlikely to occur.

A real-image finder according to a second embodiment of the presentinvention is similar to the first embodiment in the arrangement of apart thereof that extends from the objective system to the ocularsystem. The second embodiment differs from the first embodiment in thata pentagonal prism is arranged as shown in parts (a) and (b) of FIG. 7.More specifically, a pentagonal prism 73 in this embodiment hasdeflecting members 77 provided on an entrance surface 71. Eachdeflecting member 77 projects in a wedge shape. Each deflecting member77 has a pair of refracting surfaces 77′ and 77″. A ridge formed betweenthe refracting surfaces 77′ and 77″ is not parallel to an exit surface72. That is, the refracting surfaces 77′ and 77″ are in a skew relationto the exit surface 72. Therefore, no rays are deflected toward the exitsurface 72, and hence no ghost light occurs. With the real-image finderaccording to this embodiment, indications are seen as shown in part (c)of FIG. 7.

A real-image finder according to a third embodiment of the presentinvention is similar to the first embodiment in the arrangement of apart thereof that extends from the objective system to the ocularsystem. The third embodiment differs from the first embodiment in that adeflecting member is arranged as shown in FIG. 8. More specifically, adeflecting member 87 in this embodiment has refracting surfaces formedfrom diffusing surfaces 81. The deflecting member 87 has a linearconfiguration extending in a direction normal to the plane of thefigure. Consequently, deflected light rays 88 are diffused by thediffusing surfaces 81. Therefore, even if the rays 88 reach the viewer'spupil, the light intensity is weak, and ghost light is inconspicuous. Inthis case, if the prism is a molded part, it is only necessary for thesurface of a molding die to have diffusing surfaces. Accordingly, noextra processing is required, and there is no increase in the cost.

A real-image finder according to a fourth embodiment of the presentinvention is similar to the first embodiment in the arrangement of apart thereof that extends from the objective system to the ocularsystem. The fourth embodiment differs from the first embodiment in thata deflecting member is arranged as shown in FIG. 9. More specifically, adeflecting member 97 in this embodiment has refracting surfaces formedfrom curved surfaces 91. The deflecting member 97 has a linearconfiguration extending in a direction normal to the plane of thefigure. Consequently, deflected light rays 98 become diffused light.Therefore, advantageous effects similar to those in the third embodimentare obtained.

A real-image finder according to a fifth embodiment of the presentinvention is similar to the first embodiment in the arrangement of theportion that extends from the objective system to the ocular system. Thefifth embodiment differs from the first embodiment in that deflectingmembers are arranged as shown in FIG. 10. More specifically, deflectingmembers 107 in this embodiment have refracting surfaces each formed froma conical surface. Consequently, deflected light rays become diffusedlight. Therefore, advantageous effects similar to those in the thirdembodiment are obtained.

As will be clear from the foregoing description, it is possibleaccording to the present invention to provide anindication-within-finder made free from ghost light at a minimal costeven in the case of a prism having a mirror surface not included in thenormal light path in the neighborhood of an entrance surface thereof asin a pentagonal prism.

What we claim is:
 1. A real-image finder, comprising: an objectivesystem having a positive power; an image-inverting system including aprism; an ocular system having a positive power; a deflecting memberdisposed near an intermediate image plane formed by said objectivesystem to deflect incident light rays with respect to a visual fieldviewing light path; and a prism placed closer to said ocular system thansaid intermediate image plane, said prism having a mirror surface notincluded in a normal light path in a neighborhood of an entrance surfacethereof, wherein light rays deflected by said deflecting member andtotally reflected by said mirror surface do not reach said ocularsystem, thereby displaying a blacked-out indication within said finder.2. A real-image finder according to claim 1, wherein said mirror surfaceis an exit surface from which said visual field viewing light pathexits, and said light rays deflected by said deflecting member satisfythe following condition (1): 20°<ω  (1) where ω is an angle formedbetween said light rays deflected by said deflecting member and saidmirror surface.
 3. A real-image finder according to claim 1, whereinsaid light rays deflected by said deflecting member satisfy thefollowing condition (1′): 20°<ω<50°  (1′) where ω is an angle formedbetween said light rays deflected by said deflecting member and saidmirror surface.
 4. A real-image finder according to claim 1, whereinsaid deflecting member has a pair of refracting surfaces, saidrefracting surfaces satisfying the following condition (2): ω′<40°  (2)where ω′ is an angle formed between each of said refracting surfaces andsaid mirror surface.
 5. A real-image finder according to claim 1,wherein said deflecting member has a pair of refracting surfaces, saidrefracting surfaces satisfying the following condition (2′):10°<ω′<40°  (2′) where ω′ is an angle formed between each of saidrefracting surfaces and said mirror surface.
 6. A real-image finderaccording to claim 1, wherein said deflecting member has a pair ofrefracting surfaces, said refracting surfaces satisfying the followingcondition (3): 40°<θ<100°  (3) where θ is a vertex angle formed betweensaid pair of refracting surfaces.
 7. A real-image finder according toclaim 1, wherein said deflecting member has a pair of refractingsurfaces, said refracting surfaces satisfying the following condition(3′): 50°<θ<90°  (3′) where θ is a vertex angle formed between said pairof refracting surfaces.
 8. A real-image finder, comprising; an objectivesystem having a positive power; an image-inverting system including aprism; an ocular system having a positive power; a deflecting memberdisposed near an intermediate image plane formed by said objectivesystem to deflect incident light rays with respect to a visual fieldviewing light path, thereby displaying a blacked-out indication withinsaid finder; and a prism placed closer to said ocular system than saidintermediate image plane, said prism having a mirror surface notincluded in normal light path in a neighborhood of an entrance surfacethereof, wherein said deflecting member has a pair of refractingsurfaces, and a ridge formed between said pair of refracting surfaces isnot parallel to said mirror surface.
 9. A real-image finder, comprising:an objective system having a positive power; an image-inverting systemincluding a prism; an ocular system having a positive power; adeflecting member disposed near an intermediate image plane formed bysaid objective system to deflect incident light rays with respect to avisual field viewing light path, thereby displaying a blacked-outindication within said finder; and a prism placed closer to said ocularsystem than said intermediate image plane, said prism having a mirrorsurface not included in a normal light path in a neighborhood of anentrance surface thereof, wherein said deflecting member has a diffusingsurface.
 10. A real-image finder according to any of claims 1 to 9,wherein said deflecting member is provided on the entrance surface ofsaid prism.
 11. A real-image finder according to claim 10, wherein saidprism is a pentagonal prism.
 12. A real-image finder according to claim10, wherein said prism is formed by injection molding of a plasticmaterial.
 13. A real-image finder according to any of claims 1 to 9,wherein said prism is a pentagonal prism.
 14. A real-image finderaccording to claim 11, wherein said prism is formed by injection moldingof a plastic material.
 15. A real-image finder according to any ofclaims 1 to 9, wherein said prism is formed by injection molding of aplastic material.